add support for applying custom comp_op and opacity when blending map background image into background color at render time - refs mapnik/mapnik-support#8
further develop 2.3.0 changelog
more changelog additions
more 2.3.x changelog updates
Merge pull request #1966 from mapnik/background-image-comp-op

Map background-image-comp-op and background-image-opacity
add mention of background-image-opacity and background-image-comp-op to changelog - refs #1966
Merge branch 'master' of github.com:mapnik/mapnik
MinGW support for SConstruct script. Build now properly detects MinGW and finds dependencies correctly.
Properly Initialize mingwbuild variable.
Added singleton unit test to check for multiple instances across multiple dynamic libraries.
Updates for supporting 64bit MinGW build on Windows 7+ via GNU tools.
Final MinGW 64 bit enhancements for Windows 7+ and Server 2008+